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1986 xr600 dual carb rebuild

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Recently purchased a rebuild kit for my 1986 xr600 and it came with #118 and #115 mains and #45 slow jets. I've read many posts about guys running #122 jets. What gives? Just don't want to be too lean. Bike is all stock and will be used mostly trail riding in Montana.

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118s might be better for higher altitude. 122s were stock in some years and can't hurt anything but gas mileage. Generally 1 or 2 sizes bigger pilot jets seem to help starting and low speed transition though. Guideline I've  been taught many decades back was if your air screw is less than 3/4 turn the pilots too small, more than 1 1/2 your too large. Just the opposite for a fuel screw. I don't make any claims of being a jetting guru however and often consult other threads here to get all the opinions possible. When I get easy starting, clean acceleration and a brown plug I'm done. I don't need every last tenth of a horse power, that's what a 650 is for.

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That jetting is very close for XR carbs at sea level from my experience.  As VR suggested you may find it needs to be a little leaner for altitude.  I am running a 42 pilot jet which greatly improved hot restart.  For higher altitude one notch leaner would be desired on my bike as it gets harder starting hot the higher I ride. 

I run a 122/110 for the summer normally at 0-4000 feet, and that is equivalent to 118/115.  I find a richer primary give me a more linear throttle response.  Take the sum of both jets and divide by two in order to get your overall jetting.  122+110=232 /2=116.   118+115=233 /2+116.5.  They run best from my experience at 116 to 119 overall main size depending on altitude and temp.

Winter in Montana you might want a 122 primary with your 115 secondary.

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Mine is an 86, everything is stock except for a k&n style air filter element.

I think the above posts should get you in the ballpark. I tend to jet a little on the rich side to keep temps down so if most of your riding is in the 2000' range I would probably start with 120/118 or 122/120 in the mains.

For the pilot, do as posted above and size according to mixture screw.

Just my 2 cents but I wouldn't run a k&n filter on anything used in the dirt. A UNI is a much better filter.

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